This invention is concerned with improvements relating to air pressure stabilizers.
Conventionally it is the practice in a surgically clean environment to maintain an environmental pressure slightly above atmospheric pressure, to prevent possibly contaminated air from flowing into the environment, such as through gaps in doors, windows and the like. Conventionally used to control this pressure is an air pressure stabilizing device comprising a duct defining an opening, and a control member mounted in the opening for movement between open and closed positions, the control member being operative to move from its closed position in the event that pressure in the area under control rises above a predetermined level above atmospheric pressure, and to return to its closed position in the event that pressure falls below said predetermined level.
In this way, pressure within the area under control may be retained at a desired positive pressure, above atmospheric pressure.
Conventionally, the opening lies in a generally vertical plane, in which context the invention will hereinafter be described, and a conventional air pressure stabilizing device comprises a control member in the form of a generally flat plate which is mounted for pivotal movement about a horizontal axis which extends below the geometric center line of the plate, but above the center of gravity of the plate. In this manner an upper part of the plate above said axis presents to the area being controlled an area greater than that presented by the lower part of the plate. The location of the pivot axis in relation to the center of gravity ensures that, at rest, the plate is retained in a closed position. However, as the pressure in the controlled area increases, the excess force on the plate above the pivot axis causes the plate to move from its closed position, to vent the excess pressure.
Alternatively, of course, the axis may extend above the geometric center line of the plate, and above the center of gravity of the plate.
In one commercially available device, the plate is provided with a counter-balance in the form of elongate tube secured to the lower edge of the plate, which may be filled with a quantity of lead shot, to provide a means for setting the device.
However, with such an arrangement the effect of the pressure on the control member reduces as the plate opens, whilst the effect of the counter-balance increases, thus producing a steady state condition in which the angle at which the control member is inclined is proportional to the excess pressure of the area being controlled, and this arrangement has the disadvantage that an increasing excess pressure has a reduced effect in opening the control member, and this may result in the production of high velocity exiting air, which is not desired.
To compensate for this effect, and to produce an air pressure stabilizing device a control member of which opens to a greater extent in the event of an excess of air pressure in the area under control, suggestions have been made to provide a cantilevered counter-balance, so constructed that the force acting on the control member to close the control member reduces as the control member is moved from the vertical position. The effect of using two such counter-balances is to produce a force which is substantially constant throughout the movement of the control member from its vertical position towards the horizontal position. In this manner when the pressure in the area being controlled exceeds the desired maximum, the control member will automatically move from its closed to its fully opened position, allowing excess pressure from the room to be vented without the production of excessive air flow. As the pressure within the area being controlled reduces, the force acting to move the control member from its closed position reduces to a level at which it is overcome by the counter-balancing means, and the control member is returned to its closed position.
Whilst in theory such devices should operate satisfactorily, difficulties are encountered in providing such devices, particularly in enabling them to be adjusted conveniently. For example, when the control member is fully balanced, a small rise in pressure in the controlled area may cause the control member to move rapidly to its fully opened position, which may cause too much venting, and too rapid a reduction in the pressure in the controlled area, with consequent return of the control member to its fully closed position, and it is one of the various objects of this invention to provide an air pressure stabilizing device which may be utilised to control the venting of excess air from an area being controlled more satisfactorily than has hereto for been found possible.